The present invention relates to downhole seismic sources. More specifically, the present invention relates to a controlled implosive downhole seismic source.
Because the present invention is particularly useful in downhole applications, the invention will be described in relation to that use. The present invention, however, is also useful as a seismic source in other applications where power requirements, depth of operation, and hydrostatic pressure impose operating limitations on other seismic sources.
In the field of seismology, a wide variety of approaches have been used to create and detect seismic waves. It is known, of course, that seismic sources and seismic detecting means may be placed downhole or on the surface, or any combination thereof. Thus, among the various configurations, it is known to place the seismic source downhole with the detecting means on the surface, or vice versa, or with both on the surface, or with both downhole, or with the detecting means both on the surface and downhole.
It is also known that placing the source downhole contains several distinct advantages over placing the source on the surface. First, a downhole source is usually closer to the critical layers of strata which the operator is interested in, particularly in oil and gas exploration. Thus, less energy from the seismic source is lost in traveling to the desired strata layers. Second, most of the earth's surface is covered by a weathered layer of strata which consists of topsoil and fragmented bedrock. That weathered layer of strata often creates problems in sending or receiving seismic waves because of that layer's porosity. Surface sources typically put as much as two-thirds of their energy into unwanted surface or Rayleigh waves. Third, the remaining third of the energy, which contains the desired P-waves and S-waves, may be largely lost due to the porous nature of the weathered surface layers. See; Hardee, Downhole Periodic Seismic Sources, Geophysical Prospecting 31, 57-71 (1983). Fourth, a surface seismic source must also compete with other surface noises caused by man-made vibration of the earth's surface. Particularly if the testing site is near a concentrated man-made activity, such as a construction site or a highway, waves generated by a surface source will mix with those background noises and the readability of the seismic data is seriously impaired.
For those reasons, the field of seismology has long sought a downhole seismic source capable of operating under adverse conditions and yet generating acceptable seismic waves. A variety of downhole seismic sources have been suggested as attempts to achieve those goals and overcome those longstanding problems in the field. The most common type of downhole source is an explosive source. However, the explosive source has several disadvantages. First, the explosive source can damage or destroy the borehole. That result limits the use of an explosive source in the actual well bore because of the cost of potentially re-drilling the well and the potential for damage to the formation. The inability to use the source in the well bore is a significant operating limitation when seismic work is being used to map out the boundaries of a developed oil and gas field. In those cases, it is often desirable to dispose the seismic source in the well bore to find the boundaries of such structures as salt domes and subsurface inclines. Because those structures extend generally in a vertical direction, it is often necessary to determine the precise horizontal distances from an existing well at which the structures are found to avoid missing the field altogether on the next well.
A second drawback to the explosive downhole seismic source is the lack of control over the quality of the seismic source generated. Explosions, by nature, are violent reactions not prone to manipulation. It has also been recognized in the art that explosive downhole sources propagate a larger percentage of high frequency waves than is desired, but low frequency waves are usually the waves that reveal the most valuable data. See; Hardee, Downhole Periodic Seismic Sources, Geophysical Prospecting 31, 58 (1983).
In an effort to overcome the disadvantages of an explosive downhole source, a variety of prior art devices have attempted to create downhole sources which do not damage the borehole and which give the operator more control over the frequency of the waves propagated. Examples of such devices are disclosed in PCT Application No. F183/00068 and U.S. Pat. Nos. 4,038,631; 3,909,776; 3,718,205; 3,587,775; and 3,221,833. Except for U.S. Pat. No. 3,587,775, all of those devices create a seismic source downhole by mechanical operation, such as air guns, mechanical hammers, and similar devices. U.S. Pat. No. 3,587,775 discloses a controlled explosive source in which water is disassociated into hydrogen and oxygen and then ignited to provide a seismic source.
However, the devices disclosed in those references suffer from several operating limitations, including: power, depth and hydrostatic pressure.
Power is an operational limitation faced by any mechanical device. If the seismic source is powered from the surface of the borehole, additional equipment, such as an independent source of power, must be run into the hole along with the logging cable because a normal logging cable can only pass a maximum of one horsepower, which is less than the power requirements of most such mechanical devices, down a well. Explosive seismic sources are not faced with that limitation; the power supplied to the source is self contained and not dependent on any connection to the surface, except for the small amount of power needed to ignite the explosive charge.
However, unless an independent source of power is included in the device lowered downhole, problems with the second operational limitation (depth) are encountered. The mechanical devices shown in the reference mentioned above are connected to the surface by hoses or other means to provide air, fluid, or other materials required to drive those mechanical devices. If the seismic source is needed at depths over a few hundred feet, those additional cables, hoses, and other means, prove too heavy and cumbersome for practical operation. Thus, a second need is defined by finding a downhole source capable of going to significant depths, which necessitates that the source have a self contained power source.
The third operational limitation, hydrostatic pressure, is also an operating limitation for mechanical devices and explosive devices. As depth increases in the well, the hydrostaic pressure of drilling fluid, water and other materials in the borehole also increases the pressure on any seismic source placed in the well. For explosive devices, that increases means that much of the explosive power is absorbed by the hydrostatic pressure. For mechanical devices, the device must be insulated or sealed from the surrounding fluid such that the fluid will not interfere with any moving parts, air outlets, or other similar features of the device.
What the devices known in the prior art have failed to realize is that hydrostatic pressure can serve as a power source itself and does not require the addition of any hoses or other cumbersome equipment to power the seismic source.
The present invention takes advantage of that natural power source and frees the field of seismic sources from most operational limitations of depth or power. That is achieved through the present invention by providing an implosive device which is ruptured on command and which uses the power applied by the hydrostatic pressure on the fluid surrounding the device to create a seismic source. To the best of Applicant's knowledge, a controlled implosive seismic source has never been utilized in a borehole, although there have been limited experiments with implosive devices used as sources of underwater sound at shallow depths; See; Orr, M. and Schoenberg, M., "Acoustic Signatures From Deep Water Implosions Of Spherical Cavities", Journal of Acostical Society of America, Vol. 59, No. 5, pg. 1155 (May 1976), Reader, W. T. and Chertock, G. "Transient Sounds Due to Implosions of Simple Structures Under Hydrostatic Pressure", presented at 82nd meeting of Acoustical Society of America (Oct. 19, 1971), and Urick, R. J., "Implosions as Sources of Underwater Sound", Journal of Acoustical Society of America, pg. 2026 (1964). The differences between those shallow underwater experiments and a useable downhole seismic source are several.
The devices disclosed in those publications were crude experimental underwater devices and not the type of device that would be useful in a well bore. Further, those devices could not be imploded at predetermined depths, or at predetermined times, and were imploded only as a function of pressure. That limitation makes the timing of recording equipment used to detectthe source very cumbersome. Those crude devices could not be initiated remotely, and when coupled with the other factors showing a lack of control over the implosion, it is clear that those devices were not practical seismic sources because, among other factors, no more than one of those devices could be lowered into the fluid medium at a time. As explained, it is desired to be able to fire a sequence of shots when generating seismic sources.
Therefore, it is a principal object of the present invention to provide a novel apparatus for a controlled implosive downhole seismic source.
It is a further object of the present invention to gain the advantages set out above without being limited as a practical matter to the depth at which the source may operate.
It is another object of the present invention to provide a device having sufficient power to produce high quality seismic waves without damaging the borehole.
It is yet a further object of the present invention to provide a device and method able to operate under high hydrostatic pressure.
Other objects, features, and advantages of the invention will become evident in light of the following detailed description considered in conjunction with the referenced drawing of a preferred exemplary controlled implosive seismic source according to the present invention.